The invention relates to a method of preparing polymers with a low residual content of monomers.
In preparing low molecular-weight water-soluble and high molecular-weight polymers and copolymers that are water-soluble or swell up in water (partly crosslinked) it has been discovered that complete conversion of the monomers, especially monomers based on acrylic acid, is impossible. Residual contents of at least 0.5 and even 1.0% or more of free monomers are often found in polymers manufactured on an industrial scale.
Since the residual monomers are known to be toxic, a method of preparing polymers and copolymers that contain practically no monomers would be very useful. Since it has been impossible up to now to carry out polymerization in such a way as to leave no residual monomers, attempts have been made to remove the toxic residue by converting it into safe derivatives. In German Application No. 1 070 377 and U.S. Pat. No. 2,960,486, for example, aqueous solutions of high molecular weight acrylamide polymers are mixed with a solution of sodium sulfite and dried at 80.degree. to 120.degree. C. to reduce the residual content of monomers to 0.01% by weight. This approach, however, depends on processing the polymer in a very dilute (2-3%) polymer solution, which is uneconomical, and the methods have hardly been used in practice.
The monomers are directly eliminated from a polyacrylamide gel in accordance with U.S. Pat. No. 4,132,844 by heating it to 80.degree. to 150.degree. C. and in accordance with Japanese Patent Nos. 53/51289 and 50/136382 by extraction with methanol or with methanol and water, resulting in residual monomer contents of up to 0.04% by weight.
Homopolymers and copolymers of acrylamide are usually prepared today in an aqueous medium, with the polymer present either in the form of a suspension of water in oil or in that of a polymer gel with a specific residual content of free monomers. Treating a polymer gel with an aqueous solution of sodium bisulfite is described in U.S. Pat. No. 3,755,280, and with a solid alkali sulfite in European Patent No. 0 175 554, leaving residual monomer contents of 0.03 to 0.3% by weight. Japanese Patent No. 56/103207 uses bisulfites, sulfites, and pyrosulfites for the same purpose, Even sulphur-dioxide gas has been utilized to reduce the concentration of acrylamide in an emulsion polymer (U.S. Pat. No. 3,780,006).
Although the methods previously described herein do reduce the residual monomer content considerably, they have certain drawbacks, such as for example unpleasant odors due to the release of sulphur dioxide and corrosion of the plant by sulphur dioxide derivatives (e.g. sulphurous and sulfuric acids and their salts in an acidic medium).
Other methods of decreasing the residual monomer content are described with reference to the oil-in-water emulsion polymerization of the water-insoluble polymers. The residual monomer content of polybutadiene-and-styrene latex is reduced in U.S. Pat. No. 4,529,753 by adding tert-butyl-hydroperoxide and sodium metabisulfite. The residual content of ethyl acrylate is reduced from 0.43 to 0.065% by weight in Japanese Patent No. 55/151015 and German Patent No. 2 919 096 by adding ethanolamine. The monomer is removed by steam distillation in European Patent No. 002 860 and CS Patent No. 210 100 and azeotropically in U.S. Pat. No. 4,414,063. Even high-energy radiation (105 Mrad) has been exploited to reduce the residual monomer content in the manufacture of contact lenses from siloxane polymers (GB Pat. No. 2 113 694) or of polyvinyl chloride (U.S. Pat. No. 4,585,808).
The removal of residual monomers to 0.4% by weight of a polymer emulsion by catalytic hydration is described in U.S. Pat. No. 4,375,529.